1. Field of the Invention
The present invention relates generally to the a novel activated carbon that is preloaded with at least one ionic organic species and at least one metal or alkaline earth metal, such as a surfactant-iron, and the method of removing an oxyanion, such as arsenic or perchlorate, from a fluid or ground-water.
2. Description of the Related Art
During our studies on Innovative Arsenic Removal, Penn State bench-scale studies exhibited that we could efficiently and cost-effectively remove arsenic from ground water by employing a dual-vessel treatment system. The first vessel could be filled with solubilizable iron filings and the second with activated carbon that is tailored with organic-iron complexes. With such a bench dual system, we removed arsenic to below 10 ppb for 150,000 bed volumes when the influent contained 50 ppb arsenic. This system will be well-suited for small communities that aim to avoid the operational complications of conventional coagulation and filtration operations, while also avoiding the friable nature and lower sorption capacities (per unit iron) of granular iron media.
The Penn State team preloaded iron-organic carboxyl complexes onto highly porous activated carbons so as to enhance arsenic removal. We have preceded this with metallic iron filings that will be solubilized with oxygen, induced electrochemistry, and/or pH 6 water. This solubilized iron has provided a continuously fresh iron source for complexing arsenic and these complexes have been sorbed and precipitated into the tailored activated carbon.
The best results occured when ferric chloride was complexed and co-adsorbed with either citric acid, EDTA, or fatty acid. Also, favorable results occurred when the activated carbon was pre-oxidized with nitric acid, hydrochloric acid, and/or sulfuric acid, to create a considerably higher surface charge that the iron could sorb onto. It may help to include a co-complexing metal such as manganese. The competitive arsenic adsorption capacity has been tested via rapid small scale column tests (RSSCT's) for a variety of water quality conditions.
The authors anticipate that this approach will achieve cost effective and simple operations because of its several inherent advantages: (a) activated carbon systems are durable, rigid, robust, and simple to operate; whereas granular iron systems can crumble and plug. (b) iron, citrate, EDTA (ethylene diamine tetraacetic acid), fatty acids, and activated carbon are all inexpensive and non-toxic materials; (c) activated carbon hosts more surface area and pore volume per bed volume than do inorganic media. (d) The solubilizable iron bed will supply a continuously fresh source of iron to capture arsenic; and it appears that this iron more efficiently captures arsenic than does FeCl3 coagulation. We solubilized iron by corroding galvanized iron fittings; and this continuously fresh supply of Fe complexed arsenic from the passing water stream, and then the Fe—As hydroxide species sorbed and/or precipitated into the GAC media. Other sources of metal iron could be used; as could other metals or alkaline earth metals. The tailored GAC served to scavenge residual dissolved arsenic. The varying valence states that the carbon, iron, and sulfur in GAC can occur at helped to capture the arsenic and the arsenic-iron complexes.
Perchlorate appears in the groundwater that 10-20 million Americans could drink; and perchlorate might adversely affect people's thyroid gland. The present inventors have uniquely discovered how to both remove perchlorates and other undesirable anions, such as nitrates, chromates, arsenates, and arsenites, and make them available ultimately for destruction by thermally pretreating or chemically preloading granular activated carbons (GACs) prior to use. The present inventors have uniquely discovered that the process of the present invention is capable of removing perchlorates, arsenic, and other anions by a combination of removal and chemical/thermal regeneration with chemicals that are commercially available in large and inexpensive quantities and which are environmentally acceptable; and which are not known to pose health risks.
As an alternative to tailored activated carbon, Granular ferric hydroxide (GFH) is a commonly used adsorbent for arsenic removal, but these iron oxide granules can crumble and disintegrate when they experience prolonged use, whereas granular activated carbon does not crumble. Also, after backwashing, there would be significant amount of headloss pressure built up in the GFH system. Although others have loaded iron hydroxide precipitates onto activated carbon, the effectiveness of this iron was minimized because the iron was not distributed within the porous carbon, and thus the higher pounds of iron per pound of As removed was required. The inventors herein have circumvented this limitation by complexing the iron with carboxyl species. This is achieved either by complexing iron with organic carboxyl species in the water phase, and then sorbing these complexes into activated carbon, or by preconditioning the activated carbon to contain many solid carbon carboxyl species within the GAC's pores, and then sorbing iron into these pores where the solid carbon carboxyl species reside.
Arsenic causes skin cancer at low concentrations. Arsenic exceeds 10 ppb (the new arsenic standard by EPA) in at least 4000 wells that appear in more than 45 U.S. states. Many of these wells service small community water systems.
The most prevalent species of arsenic in groundwater are arsenate (+V valence) and arsenite (+III valence). The arsenate prevails in oxidized or anoxic waters, while the arsenite prevails in reduced waters that also contain hydrogen sulfide. When the water pH is between 6.75 and 11.6, the HAsO4= species will prevail in oxidized waters.
Iron, citric acid, EDTA, and fatty acids are non-toxic and commonplace in water and foods. No primary drinking water standards exist for any of these species.
The large surface area, high pore volume, and rigid structure of GAC renders it an ideal backbone for hosting a considerable quantity of iron-organic carboxyl complexes (or similar metal-organic complexes) that can be dispersed where they are available for oxyanion sorption.
Intriguingly, our Penn State system of solubilized iron and iron-organic tailored GAC has performed considerably better than any other media that the authors herein are aware of. The solubilized iron-tailored GAC system has required only 10-20 pounds of iron to remove a pound of arsenic. This stands considerably better than the 300-4000 lb Fe/lb As that the granular iron media have offered.